1. Field of the Invention
The invention relates generally to extruder screws and more particularly to extruder screws used in the production of linear low density polyethylenes.
2. Description of the Prior Art
Use of extruder screws is known in the prior art. The process of extruding utilizing a screw extruder is the most common means by which to process raw plastics into finished products. Conventional screw designs for processing plastics, such as low density (LDPE) and high density (HDPE) polyethylenes and polyvinylchlorides (PVC), have generally used square pitch flight geometries. Square pitch designs maintain a constant screw flight width, helix angle, and radial clearance, (the distance between the tip of the screw flight and the screw housing) along the length of the screw. Specifically, the conventional screw design provides a constant flight width of approximately one tenth the diameter of the screw, a constant helix angle of approximately 17.66 degrees and a constant radial clearance of approximately one thousandth of an inch per inch of screw diameter.
Recently, a new type of polyethylene plastic has been developed. The new plastic is a linear low density polyethylene (LLDPE). The LLDPE is less expensive to produce and exhibits improved mechanical properties compared to conventional polyethylene plastics. The improved mechanical properties include enhanced puncture and stress crazing resistance along with improved tensile and tear strengths. In addition, the new LLDPE plastic exhibits more newtonian-like shear characteristics than the conventional polyethylenes resulting in high levels of the polymer melt viscosity at high rates of shearing.
The processing of high viscosity plastics especially the new LLDPE plastic with the conventional extruder screw design has serious drawbacks. One disadvantage resulting from the application of conventional extruder screws to high viscosity plastic extrusion is high screw induced power consumption in the extrusion process. Such high power consumption results in low yields of plastic and high production costs.
Another disadvantage resulting from the application of conventional screw design to high viscosity plastic extrusion is the generation of high screw induced stock temperatures within the plastic, reducing overall efficiency of the extruder and increasing the manufacturing costs associated with the production of the plastics. Severe problems can occur if the plastic has limited thermal stability, as for instance in polyvinylchloride (PVC).
It is therefore an object of the present invention to provide an improved extruder screw design which reduces screw induced power consumption in the extrusion of high viscosity plastics.
It is also an object of the present invention to provide an improved screw design which reduces the high stock temperatures generated by use of a conventional extruder screw in high viscosity plastic extrusion.
Briefly, a preferred embodiment of the present invention includes an extruder screw having a variable screw flight geometry over the length of the screw. The screw flight varies with regard to its width, helix angle, and radial clearance. The extruder screw, according to the present invention, has a continuous linear taper along the length of the core of the screw. The smallest diameter end of the core of the screw is the feed section and the largest diameter end of the core of the screw serves as the metering section. The screw flight width in the feed section is one ninth of the diameter of the screw while such width in the metering section is only five percent of the diameter of the screw. The optimal metering flight width is dependent on the exact dimensions of the screw and its mechanical properties. In the preferred embodiment of the present invention the helix angle in the feed section is 20.degree. while the helix angle in the metering section is greater than 20.degree.. The optimal helix angle is dependent on the specific characteristics of the plastic to be extruded. The radial clearance in the feed section is one thousandth of an inch per one inch of screw diameter while the metering section it is five thousandth of an inch per one inch of screw diameter. Exact dimensions for the radial clearance are, as with the helix angle, dependent on the specific characteristics of the plastic being extruded.
An advantage of the present invention is that it greatly reduces the amount of extruder screw induced power consumption in high viscosity plastic extrusion thereby increasing the efficiency and output of the process.
Another advantage of the present invention is that it greatly reduces extruder screw induced stock temperature generation in high viscosity plastic extrusion thereby further increasing the overall efficiency of the extrusion process.
These and other objects and advantages of the present invention will no doubt become apparent to those skilled in the art after having read the following detailed description of the preferred embodiment which is illustrated in the drawings.